Nitrate reductase from Spinacea oleracea. Reversible inactivation by NAD(P)H and by thiols

Regulation of the nitrate-reducing system enzymes in wild-type and mutant strains of Chlamydomonas reinhardii

Six mutant strains (301, 102, 203, 104, 305, and 307) affected in their nitrate assimilation capability and their corresponding parental wild-type strains (6145c and 21gr) from Chlamydomonas reinhardii have been studied on different nitrogen sources with respect to NAD(P)H-nitrate reductase and its associated activities (NAD(P)H-cytochrome c reductase and reduced benzyl viologen-nitrate reductase) and to nitrite reductase activity. The mutant strains lack NAD(P)H-nitrate reductase activity in all the nitrogen sources. Mutants 301, 102, 104, and 307 have only NAD(P)H-cytochrome c reductase activity whereas mutant 305 solely has reduced benzyl viologen-nitrate reductase activity. Both activities are repressible by ammonia but, in contrast to the nitrate reductase complex of wild-type strains, require neither nitrate nor nitrite for their induction. Moreover, the enzyme from mutant 305 is always obtained in active form whereas nitrate reductase from wild-types needs to be reactivated previously with ferricyanide to be fully detected. Wild-type strains and mutants 301, 102, 104, and 307, when properly induced, exhibit an NAD(P)H-cytochrome c reductase distinguishable electrophoretically from constitutive diaphorases as a rapidly migrating band. Nitrite reductase from wild-type and mutant strains is also repressible by ammonia and does not require nitrate or nitrite for its synthesis. These facts are explained in terms of a regulation of nitrate reductase synthesis by the enzyme itself.

Metabolism of hydroxylamine by spinach leaf discs and its relationship to nitrate reduction

Nitrate reduction in vivo by spinach leaf discs was shown to be inhibited by hydroxylamine when this was included in the nitrate reductase assay solutions or introduced to the tissue during a preincubation period. The sensitivity of nitrate reduction to hydroxylamine was not sufficient to suggest a natural process, considering the small endogenous concentrations of hydroxylamine in the leaves. Inhibition of nitrate reduction in vivo could be approximately related to rates of in vitro inhibition of nitrate reductase by this compound. There was no need to suppose conversion of hydroxylamine to cyanide to inhibit nitrate reduction. Some of the in vivo and in vitro characteristics of hydroxylamine inhibition of nitrate reductase are described. Hydroxylamine was metabolised by discs at rates comparable to nitrate reduction. Rates of metabolism of hydroxylamine, and its accumulation in the tissues from an external solution were both enhanced by light but little affected by anaerobiosis.

The reactivation of nitrate reductase from spinach (Spinacia oleracea L.) inactivated by NADH and cyanide, using trivalent manganese either generated by illuminated chloroplasts or as manganipyrophosphate

Nitrate reductase of spinach (Spinacia oleracea L.) leaves which had been inactivated in vitro by treatment with NADH and cyanide, was reactivated by incubation with oxidant systems and measured as FMNH2-dependent activity. Reactivation was produced with trivalent manganese compounds represented either by manganipyrophosphate or produced by oxidation of Mn(2+) ions in the presence of illuminated chloroplasts and compared with reactivation obtained with ferricyanide. Reactivation in the chloroplast system was equivalent to that with ferricyanide when orthophosphate was used but was variable and weak in the presence of pyrophosphate, although manganipyrophosphate was formed, freely. Reactivation by manganipyrophosphate in dark reaction conditions was less effective than with ferricyanide but was not inhibited by the addition of pyrophosphate. Reactivation with illuminated unheated chloroplasts was dependent on added manganese and oxidation of manganese in the presence of pyrophosphate was abolished by boiling the chloroplasts. In the presence of orthophosphate however, boiled, illuminated chloroplasts reactivated the enzyme in the absence of added manganese. Reactivation occurred spontaneously in air, more slowly than with the other oxidants, but to a similar extent to that produced by manganipyrophosphate. The results provide a possible model for physiological reactivation mechanisms.